In a thin film transistor (TFT) employing a metal oxide semiconductor, an insulating protection layer (etch stopper) is typically formed on the semiconductor layer in order to ensue the reliability of the channel portion. In this case, the source and drain electrodes are connected to the semiconductor layer via two connection holes (contact holes) which are formed in the protection layer.
In the structure, however, it is necessary to highly precisely form the two contact holes with a predetermined clearance, and therefore, it is disadvantageously difficult to provide a high-precision small transistor. It is also disadvantageously difficult to ensure a stable connection using the minute contact hole. Moreover, at a portion where the metal oxide semiconductor and the electrode are joined together, if oxygen contained in the metal oxide semiconductor migrates to the electrode, an oxide is formed on the surface of the electrode and acts as an electrical barrier, so that an appropriate ohmic contact may not be ensured.
To address the above problems, a TFT has been described in which the channel portion, the source portion, the drain portion, and the pixel electrode are formed of a metal oxide semiconductor, and the resistance of a partial region including the contact hole of the metal oxide semiconductor film is reduced (PATENT DOCUMENT 1). PATENT DOCUMENT 1 also describes a bottom-gate TFT as an application of the TFT and the user of InGaZnO4 as the metal oxide semiconductor.
PATENT DOCUMENT 1 also describes that in the metal oxide semiconductor film, oxygen vacancies in the film act as electron donors, that if oxygen is removed, electrons left in oxygen vacancies act as carriers through the semiconductor, and that the conductivity of the metal oxide semiconductor can be reduced by a plasma treatment etc.
In the TFT, after an uppermost protection insulating film is formed, an opening is formed in the protection insulating film so that a part of the channel portion etc. is exposed to reducing plasma through the opening, whereby a low-resistant region is formed in the channel portion etc.
PATENT DOCUMENT 1 also describes that, in order to provide a satisfactory ohmic contact between the source signal line and the metal oxide semiconductor, the metal oxide semiconductor film is formed into a desired island shape by patterning, and thereafter, a hydrogen plasma treatment is performed, and thereafter, a source metal film is formed on the metal oxide semiconductor film, and patterning is performed on the source metal film to form the source signal line.
Note that the hydrogen plasma treatment and the source metal film formation are successively performed without being exposed to the atmosphere, and thereafter, a protection insulating film is formed on the source signal line and the metal oxide semiconductor film. The plasma treatment is performed after this series of processes.